general automotive suspension spring material question

[WillCole]

Forgive the amateurish question, but could someone tell me why automotive suspension springs are generally silicon steels?

It seems to me that higher tensile steels would increase fatigue life.

My only guess was that heat build up in the spring required a more heat tolerant material? I would be interested in any insight.

 

[TVP]

High strength steels for automotive applications like suspension coil springs and engine valve springs are alloyed with high amounts of silicon because it confers increased strength and hardness (solid solution hardening), higher sag resistance (resistance to load loss, resistance to stress relaxation) and temper resistance (resistance to softening during tempering and stress relieving). I'm not sure what you mean by the comment about higher tensile steels-- contemporary spring steels are quenched and tempered to very high strength (1900-2150 MPa, 53-57 HRC, 560-640 HV). Here are some links that explain the effect of Si on spring steels, with some of the content specific to valve springs, but the compositions and processing are very similar for suspension springs:

http://www.kobelco.co.jp/english/ktr/pdf/ktr_26/021-025.pdf

http://www.kobelco.co.jp/english/ktr/pdf/ktr_27/023-027.pdf

http://www.kobelco.co.jp/english/ktr/pdf/ktr_27/075-075.pdf

http://www.vanitec.org/pdfs/b4456fcc4cffb685b9f8b171aeb8a467.pdf

https://tspace.library.utoronto.ca/bitstream/1807/12732/1/MQ45601.pdf

http://www.postech.ac.kr/~cslee/k/literature/inter/24.pdf

http://141.223.168.124/result/papers/int/43e.pdf

http://141.223.168.124/result/papers/int/33e.pdf

http://www.cbmm.com.br/portug/sources/techlib/science_techno/table_content/sub_4/images/pdfs/042.pdf

 

[WillCole]

TVP - First let me thank you for that wealth of information. I suspect I might have a misunderstanding. I looked through the Handbook of spring design published by SMI and I noticed that fatigue resistance seemed to be a direct function of tensile strength. I then came across the statement:

"Music Wire - This is the most widely used of all spring materials for small springs because it is the toughest. It has the highest strength tensile and can withstand higher stresses under repeated loading conditions than any other spring material. It can be obtained in diameters from 0,12 to 3mm. It has a usable temperature range from 0 to 120C"

And this is where I likely went astray... My reasoning went if music wire has a higher tensile, but lower temperature tolerance perhaps they use silicon steels because the thicker springs build up heat and pass the 120C internally.

I did notice in the first article you posted that there is a point of diminishing returns when it comes to hardness and fatigue resistance. This is slight confusing because of the reference to nitriding the surface of that springs.

So it appearing to me that tensile is part of the equation, but after a certain point grain structure becomes more complicated. Or am I off track and the issue is related to wire size?

 

[swall]

Sag resistance, aka "taking set" is the main reason for the prominence of Cr-Si steels for suspension springs. Silicon steels are superior to plain carbon steel wire and alloy steel wire in that respect--look at Figure SM-1 in the SMI Handbook. Most,if not all of the specialty spring steels, as described in the links supplied by TVP, start out as Cr-Si steels and are tweaked via other alloy additions to suit additional needs, such as resistance to softening during nitriding. The tensile strength advantage of music wire over Cr-Si diminishes as the wire size increase and after 1.5mm, Cr-Si has the advantage--see Figure SM-3 in the handbook. Most suspension springs are shot peened and any limitations that Cr-Si may have relative to fatigue resistance compared to music wire is offset by proper peening.

 

[israelkk]

Music wire maximum wire diameter is 6 to 7 mm because its strength is due to cold drawing. While silicon spring steels are heat treated therefore, they are not limited to small wire sizes. Most automotive suspension springs needs larger wire diameter springs.

 

[WillCole]

Thank you everyone who replied. The wire thickness issue makes a lot of sense. Also the greater tolerance for surface treatments.

I also hadn't properly considered the issue of sag as an independent from a fatigue fracture. With suspension springs you have a lot of geometry to play with (relatively speaking) so you can design a high tolerance to fracture failure, but you still have to deal with the car riding lower and lower as the springs 'wear'.

I guess that leads me to the next question as inspired by the articles provided by TVP. The first article makes a comment about spring fatigue resistance plateau'ing about 1800 MPa.

I am inferring that is specifically for wire relevant to valve springs. I was of the understanding that for the straight carbon steel wires (of smaller diameter) there is advantage up to considerable higher numbers. Again, if I am misunderstanding I would welcome any education folks care to provide.